Flow boiling characteristics in different configurations of stepped microchannels

Geometrical modifications in microchannel geometry intended towards mitigation of flow instabilities generally results in increased pressure drop across the microchannel heat sink. To overcome this, stepped microchannels configurations with combination of narrow bottom (aspect ratio: AR = depth/width > 1) and wider top (AR < 1) part are investigated and their flow boiling performances have been compared with rectangular cross-section microchannels (RMC) having same hydraulic diameter (350 mu m). The effect of varying wider cross-section on flow boiling instability and heat transfer performance have been explored for three configurations of stepped microchannels namely: stepped straight microchannels (SMC), stepped converging microchannels (SCMC) and stepped diverging microchannels (SDMC). These geometrical variants are fabricated on a 10 x 10 mm(2) copper block having nine parallel microchannels. Flow boiling experiments have been conducted using de-ionized (DI) water as coolant for an inlet subcooling of 20 degrees C, coolant mass fluxes ranging from 327.6 to 777.54 kg/m(2) s and for wide range of heat fluxes. Combination of narrow and wider converging/diverging flow passage improves flow stability by allowing bubbles in the confined narrow bottom to grow and expand in the wider top section thus reducing their evaporative momentum. Different flow regimes govern the heat transfer and pressure drop mechanisms while the compressible volume-liquid interaction dictates flow instability. Favorable surface tension gradient in SDMC alleviates flow instability whereas reduced flow resistance at the inlet in SCMC resulted in smooth rewetting during dry out. Amplitude and fluctuation of pressure drop is found to be minimum in SCMC among all geometric configurations, while better heat transfer performance has been observed in SDMC configuration.